1. Field of the Invention
This invention relates to A method of estimating the clearance distance between a magnetic read/write transducer and a magnetic recording disk.
2. Description of Related Art
In data storage devices such as magnetic disk drives, both hard and floppy, and in tape storage systems, the magnetic transducer head is positioned over the magnetic medium whilst data is read from or written onto the magnetic medium. Higher recording densities are obtainable by reducing the clearance between the magnetic transducer head and the magnetic medium. Magnetic recording technologies are achieving data storage densities in they region of 1 Gb/in2 and greater. However, with such high recording densities, it is difficult to maintain the low constant clearance required between the magnetic head and the magnetic disk.
Generally, a typical disk in a disk drive comprises a substrate, an underlayer, a layer of magnetic medium, a protective layer, and a layer of lubricant which comprises the effective surface of the disk. The tern head-disk clearance is the distance between a magnetic transducer head and a magnetic disk.
The phenomena of contamination build-up was not as serious as it is now because the head-disk clearance was in the region of 100 nm or greater, and a build-up of contamination on the transducer head did not affect the head-disk clearance significantly. Forehead-disk clearances in the xe2x80x9cnear contact regionxe2x80x9d where the magnetic read/write transducer has a chance of contacting the disk surface, the clearance between the magnetic read/write transducer and the magnetic disk is in the region of 15 to 40 nm. At such a small head-disk clearance, the chance of the lubricant portion of the disk coming into contact with the surface of a slider or being transferred from the disk surface to the slider increases significantly. It has been found that contaminant on the transducer can be as thick as 30 nm or more. Such build-up requires an increased head-disk clearance and as a result, the read/write performance will be degraded. Since disk drives are now being routinely manufactured to operate with the head-disk clearance within the near-contact region, there is a need for a method of estimating the variation of the clearance between the magnetic read/write transducer and the magnetic disk so that variations on the disk surface due to contaminant build-up, which could result in excessive degradation of the recording performance, can be detected so that start/stop operations can be initiated to remove or reduce the contamination build-up.
A number of methods are known for measuring the clearance between magnetic read/write transducers and a magnetic recording medium. For example, U.S. Pat. No. 4,872,071 describes a method and apparatus of detecting abnormal operation of a magnetic disk drive by reading data from a pre-determined area of each track. The amplitude of the readback signals from each track are compared with a corresponding reference level. If the comparison indicates that the variation in the clearance is above a predetermined level, then an alarm signal is provided so that necessary actions can be taken to avoid the possibility, of a head crash. However, this method does not take into account that the amplitude of the readback signals varies when the magnetic head slider is not located perfectly along the center line of a track.
U.S. Pat. No. 4,777,544 describes a method and apparatus of the in-situ measurement of the clearance between a magnetic read/write transducer and a magnetic medium. A pre-written signal located at a predetermined area of the magnetic recording medium is readback at two different clearance levels. The Wallance Equation is used to measure the variation of the clearance. In a further embodiment, a signal is recorded which has a spectral content comprising a plurality of different frequencies, and readback signals are simultaneously sensed at two separate wavelengths. A drawback of this method is that it is not convenient for measuring the variation of clearance between a magnetic read/write trasducer and a magnetic disk during normal data read/write operations.
U.S. Pat. No. 5,130,866 describes a method and circuitry for in-situ measurement of a magnetic read/write transducer and a magnetic medium clearance. Isolated transitions are pre-written on a magnetic medium. During a readout process, the width of a readout signal of an isolated transition increases as the clearance increases. The pulse width is measured during a reading operation and is used to indicate the in-situ variation of clearance. However, a drawback of this method is that the electronics required to implement this method are quite complicated compared with the read channel of present disk drives and are not easy to build into a disk drive system.
It is an object of the present invention to provide a simple in-situ method for detecting variations in the clearance between a magnetic read/write transducer and a magnetic recording disk in an operational magnetic disk drive or tape storage system.
Accordingly, the present invention provides a method of detecting variations in a clearance distance between a magnetic read/write transducer and a magnetic recording medium having a clearance detection burst recorded thereon, the clearance detection burst comprises at least two groups of bit patterns having different transition densities, the method comprises the steps of: obtaining a readback signal from the at least two groups of bit patterns, the readback signal being sensitive to variations in the clearance; detecting the amplitude of the readback signal of a first group; detecting the amplitude of the readback signal of a second group; comparing the amplitudes of the readback signals from the at least two groups of bit patterns to provide an indication of a variation in the clearance.